1. Field of the Invention
The present invention relates to a power transmission apparatus, having a torque limiting function, which is particularly useful for a compressor, for an air conditioner in a vehicle, which is always driven by an external power source, such as an engine, through a belt or the like.
2. Description of the Related Art
A refrigerant compressor for an air conditioner in a vehicle and, in particular, a car air conditioner, is driven by an external drive source, such as an engine, through a belt and a pulley, etc. An electromagnetic clutch may be provided between the compressor and the engine to selectively establish a connection therebetween. However, an electromagnetic clutch is often not provided to reduce the manufacturing cost. In the absence of an electromagnetic clutch, the power transmission mechanism is provided with a torque limiter to prevent the engine from being influenced by trouble on the compressor side.
FIG. 11 shows a known power transmission apparatus with a torque limiter for use with a car air conditioner. FIG. 12 shows an enlarged view of a sleeve ring of a power transmission apparatus. In FIG. 11, the metal sleeve ring 5 which is formed integrally with a resin pulley 1 by insert-molding is provided on its rear end (right end in FIGS. 11 and 12) with an inwardly bent portion 5b. A bearing 7 is inserted in the sleeve ring 5 from the front side (left side in FIG. 11) of the sleeve ring 5 until the bearing abuts against the bent portion 5b. After that, the front end of the sleeve ring 5 in FIG. 11 is calked to form a calked portion 5a to thereby secure the bearing 7 to the sleeve ring 5. Thus, the bearing 7 is attached to the pulley 1. However, in this arrangement, if an external load is applied in the forward direction of the pulley, the calked portion 5a receives the load, so that sufficient calking strength cannot be obtained. As a result, the calked portion is loosened, and consequently, the position of the bearing 7 relative to the sleeve ring 5 and accordingly to the pulley 1 deviates, resulting in breakage of the bearing, etc.
Furthermore, in the case of a pulley made of a metal, heat generated in the bearing 5 is passed to the air through the metal pulley. However, in the case of a conventional resin pulley, heat generated in the bearing 5 tends not to be radiated through the pulley made of resin, thus resulting in reduction of the durability of the bearing.
To this end, in the prior art, the sleeve ring integrally formed with the resin pulley by insert-molding can have an extension extending from the portion that is in contact with the bearing to define a heat radiating portion which is exposed to the air, as disclosed in Japanese Unexamined Patent Publication No. 2001-227620.
However, in the resin pulley as disclosed in JPP '620, the extension of the sleeve ring in the axial direction to provide a large heat radiating surface area of the sleeve ring increases the thickness of the entire pulley, thus leading to an increase in the size of the power transmission apparatus. In order to decrease the thickness of the pulley, if the rear portion of the sleeve ring is made larger in the radial and outward direction, as shown in FIG. 13, a pulley thickness t large enough to ensure the strength of the resin pulley cannot be obtained.